As well known in the art, bonded magnets have many advantages such as light weight, good dimensional accuracy, and facilitated mass-production of molded products having even a complicated shape as compared to sintered magnets, and, therefore, have been extensively used in various applications such as toys, office equipments, audio equipments and motors.
As the magnetic particles usable in the bonded magnets, there are known rare earth element magnet particles such as typically Nd—Fe—B-based alloy particles, or ferrite particles. The rare earth element magnet particles have high magnetic properties, but are expensive, resulting in limited applications thereof. On the other hand, the ferrite particles are somewhat deteriorated in magnetic properties as compared to the rare earth element magnet particles, but are inexpensive and chemically stable and, therefore, have been used in more extensive applications.
The bonded magnets have been in general produced by kneading a rubber or a plastic material with magnetic particles and then molding the resulting kneaded material in a magnetic field or by using a mechanical means.
In recent years, with the enhancement in performance of various materials or equipments including an improved reliability, there is also an increasing demand for a high performance of bonded magnets used therein including enhancement in strength and magnetic properties of the bonded magnets.
More specifically, the bonded magnet molded products obtained by injection molding, etc., are also required to exhibit a magnetic potential inherent to magnetoplumbite-type ferrite particles packed therein to a maximum extent. That is, since the ferrite particles have such a feature that they are highly oriented against an external magnetic field, the bonded magnet molded products are capable of realizing a high magnetic force and a complicated multipolar waveform.
For example, in the applications of motors, rotors and sensors, the bonded magnet tends to be frequently subjected to multipolar magnetization when machined into various sizes and complicated shapes by injection molding. For this reason, in order to satisfy the multipolar magnetic waveform and magnetic force as desired, it has been strongly required that the ferrite particles exhibit a high orientation during flowing of the resin composition.
In the motors and rotors, when feeding a large amount of electric current through an exciting coil, a large diamagnetic field is applied to a magnet, so that a residual magnetic flux density Br of the magnet is reduced by from several % to about 10-odd % owing to demagnetization thereof. Therefore, in the bonded magnets used in the motors and rotors, it is necessary to take the demagnetization owing to the diamagnetic field into consideration, and the bonded magnets are required to have a high coercive force as well as a reduced demagnetizing factor. In this case, the diamagnetic field in which the residual magnetic flux density Br of the bonded magnets is reduced up to 0 mT owing to demagnetization thereof is represented by a coercive force iHc, whereas the diamagnetic field in which the residual magnetic flux density Br of the bonded magnets is reduced by 10% owing to demagnetization thereof is represented by Hk. As Hk of the bonded magnets is increased, the degree of demagnetization thereof when used in the motors and rotors becomes smaller. Therefore, it is necessary to enhance Hk as an index of demagnetization resistance of the bonded magnets in the motors and rotors. That is, it is necessary to improve squareness of of the bonded magnets.
For this reason, the ferrite particles used in the bonded magnets as well as the resin compositions for the bonded magnets which comprise the ferrite particles and an organic binder are also required to satisfy the above requirements.
Conventionally, ferrite particles for bonded magnets and resin compositions for bonded magnets which comprise the ferrite particles and the organic binder have been improved variously. For example, there are known the method of producing ferrite particles by using an alkali metal compound or an alkali earth metal compound as a flux (Patent Literature 1); the method of using anisotropic ferrite particles and an inorganic substance pulverized product (Patent Literature 2); the method of producing a bonded magnet using ferrite magnetic particles comprising an alkali earth metal as a constituting component and having an average particle diameter of not less than 1.50 μm and a melt flow rate of not less than 91 g/10 min (Patent Literature 3); the method of controlling properties of compacted calcined particles obtained by producing particles having an average particle diameter of not more than 2.5 μm and a specific surface area of not less than 1.25 m2/g and then subjecting the resulting particles to annealing and further to compaction, so as to satisfy the conditions of Ra<2.5 μm and Ra−Da<0.5 μm wherein Ra (μm) represents an average particle diameter of the particles as measured by a dry air dispersion laser diffraction method, and Da (μm) represents a specific surface area diameter of the particles as measured by an air permeability method (Patent Literature 4); the method of calcining a ferrite at a temperature of 1050 to 1300° C. under a saturated vapor pressure of a chloride thereof, mixing the calcined ferrite with fine ferrite particles having a small particle diameter, and then annealing the resulting mixture at a temperature of 800 to 1100° C. to obtain a ferrite having a large particle diameter, a clear crystal structure, a coercive force that is hardly reduced even when pressed, and an energy product of not less than 2.0 MGOe (Patent Literature 5); or the like.